Radiation therapy treats tumorous tissue by delivering a prescribed lethal dose of high-energy radiation to the tumorous tissue. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed and that damage to the surrounding and adjacent non-tumorous tissue is minimized. One type of such therapy, known as external-beam radiation therapy, uses a radiation source that is external to the patient. The source is typically either a radioisotope, such as cobalt-60, or a high-energy x-ray or electron source, such as a linear accelerator. The external source produces a collimated beam of radiation that is directed at the patient, toward the tumor site.
Accurate measurement of the radiation intensity profile of a radiotherapy beam is important in radiation therapy quality assurance, radiation therapy treatment planning or verification of treatment. The radiation intensity profile, as its name suggests, is a description of the characteristics of a radiotherapy beam, such as the sizes of the beam center, penumbras of the beam, and overall field, the degrees of flatness and symmetry of the beam, intensity (i.e. energy output) of the beam at given points, and/or other features. “Beam profile” generally indicates the size, shape or distribution of a beam, and “intensity” generally indicates the energy output of the beam. The ability to precisely characterize the radiation intensity profile also permits evaluation of any filters or attenuating blocks that may be used in conjunction with the radiation beam to shape or attenuate the radiation beam.
Several methods may be used to determine the intensity profile of a radiation beam. In one method, the beam is linearly scanned using a small radiation detector, typically an ionization chamber, within a tank of water, which simulates the patient radiation absorption characteristics. Another similar method uses a diode in place of the ion chamber and utilizes plastic material placed above the detector instead of water. The beam is scanned on an axis that is perpendicular to the beam axis, typically passing through the central axis of the beam. Some systems utilize a second detector, usually placed at iso-center, which is used to compensate for variations in beam intensity. In all beam-scanning systems, the detector generates a current that is proportional to the radiation intensity at each point in the scan. Typical water tank scanning systems include models MP3, MP3-S, MP3-XS, MP2, MP1, MP1-S, Type 4322, Type 41001, and Type 41014 manufactured by PTW Freiburg; Blue Phantom, RFA-200, RFA-300, and 2-D Phantom made by Wellhöfer; DynaScan Model 3112 manufactured by CMS; and Model 3000 and Model 2000 made by Advanced Radiation Measurements (ARM). A typical solid-state (diode) scanning system is the Nuclear Associates BeamScan.
This method has been widely accepted and offers several advantages over other methods. One of the primary advantages is the high spatial resolution (i.e. the ability to obtain precise readings) along the scanned axis, which is typically determined by the mechanical resolution of the scanning device or mechanism. Since only one or two detectors are used, another advantage is the low number of detectors and their associated readout electronics (electrometers). This reduces circuit complexity and limits the number of detectors that are required to be calibrated. Many of these systems offer beam scanning in three dimensions.
This method of beam scanning also has several limitations and drawbacks. One limitation concerns the measurement of dynamic radiation treatment modalities, such as wedge-shaped radiation dose distribution (“dynamic wedge”) or intensity modulated radiation therapy (IMRT), in which the radiation intensity and/or beam profile changes over time. Quality assurance within such modalities requires measurements at many different points in the dynamic treatment to accurately characterize the beam dynamics over time. However, to accurately measure the beam profile using existing beam scanning methods as noted above, the beam profile must be static and its intensity must be constant during the scan, i.e. the beam cannot change over time. Variations in the beam profile or intensity during the time needed to scan the beam result in errors in the measured beam profiles. Accordingly, numerous separate measurements of radiation intensity profile, representing different points of static beam profile and constant intensity, must be made to fully characterize a dynamic treatment modality. A significant amount of time is generally required to make these measurements, which can become tedious. Further, operating a radiotherapy source for such extended time periods may also place undue stress on the source's and detector's components. The beam scanning system must be mechanically aligned properly and its positioning mechanisms must be accurate and reproducible. In addition, setting up and taking down a water tank and/or other beam scanning device can also be very time consuming and inconvenient.
A second method of measuring a beam's radiation intensity profile uses multiple detectors, such as ion chambers or diodes, in a linear array so as to simultaneously measure the beam intensity at numerous measurement points. Using this method, the detector array is placed perpendicular to the beam axis, typically passing through the central axis of the beam. Each detector generates a current that is proportional to the radiation intensity at its position in the array. Array detectors may be available with beam scanning devices that allow them to scan a beam to create a two dimensional beam profile image.
Making simultaneous measurements of radiation intensity profile with an array of detectors offers several advantages over the previously described beam scanning method. Since acquisition of data concerning the beam profile occurs simultaneously for all detectors in the array, several points on the beam profile are measured simultaneously to produce a “snapshot” of the beam profile at a specific moment in real time. This eliminates the need for multiple static beam profile measurements at many different points in a dynamic treatment. As a result, beam profile measurements may be made in real time for dynamic treatment modalities. This reduces stress on the radiotherapy source and is less tedious than the beam scanning method. It is often easier to set up and more convenient.
The limitations and disadvantages of this method result from the number of locations in the beam that must be sampled and the small size of the detectors required in order to generate an image of the beam profile with suitable spatial resolution. Moreover, simultaneous measurements by a detector array require readout electronics for each detector, typically an electrometer, to process the signal from each detector. It is known to place the detectors and readout electronics in close proximity on the same printed circuit board. In that scheme, however, radiation strikes both the detectors and the electronics, resulting in damage to the electronics that occurs with accumulated radiation dose. It is also known to place an interconnecting cable between the detectors and readout electronics, allowing the readout electronics to be located a distance from the radiation beam, significantly reducing the amount of radiation damage to the electronics. Collecting the small currents from a large number of detectors requires multiple coaxial low noise cables (or a multi-conductor cable with a conductor for each detector) to transfer the small detector signals to the readout electronics. However, the size and lack of flexibility of multi-conductor cables, or the number of individual cables, make such a system fairly cumbersome. The requirement for separate readout electronics for each detector also adds considerable cost to the system.
The spatial resolution of a detector array is also limited by the size of the detector. In an ion chamber array, for example, the detector size is limited by the signal-to-noise ratio of the detector. Since the current produced by an ion chamber detector is proportional to its volume (in a constant radiation flux), a smaller ion chamber produces a smaller signal and signal-to-noise performance is degraded. Accordingly, the ion chambers in an array cannot be so small that the noise overcomes the signal to the extent that the signal cannot produce a resolvable output.
The calibration requirements of a detector array also are a disadvantage. Each detector and its associated readout electronics must be individually calibrated in order to provide an accurate representation of a beam's radiation intensity profile. Additionally, all detectors must respond in the same way to variations in beam energy and intensity, to ensure accurate and precise readings for dynamic radiation treatment modalities.
Further, currently most detector arrays are not waterproof and cannot be used in a water tank to obtain beam profiles and intensities at various depths of water. Radiation beam quality assurance using a water tank to simulate treatment conditions is a well-known practice. Instead, to use non-waterproof detector arrays acrylic or plastic elements are stacked on top of the array to simulate water. Existing ionization chamber array systems include Thebes Model 7000 made by Victoreen; CA-24 made by Wellhöfer; and LA-48 made by PTW Freiburg.
When diodes are used as detectors for either of these beam-profiling methods (beam scanning or simultaneous measurements), their energy response changes and their sensitivity degrades with accumulated dose and must be corrected through frequent recalibration. The diodes must then be replaced when this degradation becomes significant. Existing solid-state detector array systems include Model 1170 Profiler and Model 1172 Solid-State Dosimetry (SSD) System made by Sun Nuclear; Hi-pSi Semiconductor Array made by Wellhöfer; and BMS 96 made by Schuster.
A third method of measuring the beam's radiation intensity profile uses film that is sensitive to x-ray or gamma radiation and a film scanner or densitometer to measure the optical density at many points on the exposed film. The exposed film density is proportional to the film's accumulated radiation exposure. To perform beam measurements by this method, the film is placed between layers of material whose radiological properties are similar to human tissue or water. The film is then placed at the desired position in the radiation beam, exposed, processed (if necessary) and scanned by a film scanner. Software is then used to convert the scanned results into a radiation intensity profile. Alternatively, a densitometer may be used to perform the film density measurements manually.
The primary advantage of this method is that it captures a high-resolution image of the beam intensity profile in two dimensions simultaneously. This allows a single film to be used to produce a two dimensional image of a static beam profile or an integrated two-dimensional image of a dynamic treatment profile.
This method has limitations when used for measurements of dynamic treatment modalities in which the radiation intensity and/or beam profile changes over time. This requires measurements at many different points in the dynamic treatment to accurately characterize the beam dynamics over time. In order to accurately measure the beam profile, the beam profile must be static and its intensity must constant during the film's exposure time. Variations in the beam profile or intensity during this time can result in errors in the measured beam profiles. A significant amount of time is generally required to expose, process and scan the films, which can become tedious. Operating a radiotherapy source for the extended time periods necessary to fully characterize a dynamic treatment may also place undo stress on its components. Further, accurate registration of the film with the beam is required to accurately translate the positional information from the film into actual positions within the beam. The film must also be aligned properly and its positioning must be accurate and reproducible. Like most detector arrays, however, film is not waterproof and typically cannot be used in a water tank to obtain beam profiles at various depths of water. Instead, film is stacked between acrylic or plastic water plates to simulate water. Typical film dosimetry systems include the DynaScan Model 1710 Laser Densitometer made by CMS; and the RIT 113 Radiation Therapy Film Dosimetry System made by RIT with a Vidar or Lumisys film scanner.
Accordingly, there is a need for an apparatus and methods for detection of radiation that is convenient, inexpensive, easy and quick to operate, and will limit the degradation resulting from contact with radiation.